When two or more reactants are reacted, any local heterogeneity in reactant concentration is likely to cause formation of by-products. A heterogeneous concentration can cause non-uniformity in the particle size and crystal form of the product precipitated in a reaction system in the form of fine crystal particles. Thus, it is necessary to produce a compound at a high yield using a simple process. Also, in order to produce a crystalline compound with a non-uniform particle size using a simple process, efficient mixing of reactants is needed.
In a mixing process, reactant fluids are forcedly stirred by laminar or turbulent flow, further diffusing reactant molecules mutually. According to the Fick's law of diffusion, the diffusion time is proportional to the square of diffusion distance, so the diffusion time can be reduced by decreasing the diffusion distance. More specifically, in a mixing process, the mixing rate at a molecular level can be increased considerably by forcibly stirring two or more reactant fluids to thereby divide them into fine segments and bring the segments into contact with each other.
In recent years, microreactors have attracted attention as a means for increasing the mixing rate. The microreactor means a chemical reaction device utilizing the interior of a microspace of several to several hundred micrometers. With the use of the microspace, the surface area of reactants per unit volume of a reaction system as well as the area of contact between the reactants become greater, so that effective mixing and interface reaction can be effected. Further, in standard, non-microreactor plants, stirring efficiency varies greatly with the increase in scale, so the reaction conditions need to be reconsidered when those plants are scaled up. On the other hand, the production scale of microreactors can be expanded not by “scaling up” (increasing the size) but by “numbering up” (increasing the number of reactive sites); thus, a rapid shift from research and development to industrial production can be achieved. In reality, however, there is a limit on the expansion of the production scale of microreactors by the numbering up approach; accordingly, there has been a need for a new device and method.
The membrane reactor which supplies liquid b to liquid a through a porous membrane to react them is expected to be a device that is effective for expanding production scale while maintaining excellent mixing performance. A known membrane reactor is exemplified by a reactor that uses a cylindrical porous membrane made of Shiras porous glass (hereinafter also referred to as “SPG membrane”) (Non-patent Document 1). In this type of membrane reactor, liquid a is supplied into a cylindrical porous membrane such that the streamline forms a straight line parallel to the longitudinal direction of the cylinder, and liquid b is supplied to liquid a through the porous membrane. In other words, liquid a is caused to flow orthogonally to the flow of liquid b and is thus also referred to as a “cross-flow liquid.” The cross-flow liquid is susceptible to being eliminated from the surface of the membrane by the flow of liquid b. This results in retention of liquid b in the vicinity of the membrane surface, leading to a decrease in the efficiency of its mixing with the cross-flow liquid.
Thus, Yong Wu, et al., proposed that, in order to increase the mixing efficiency, static stirrers of different shapes should be placed in the cylinder made of a cylindrical porous membrane to effectively stir a cross-flow liquid (Non-patent Document 2). However, this method involves difficulty in precisely controlling the flow of the cross-flow liquid in the boundary layer on the membrane surface and does not show an adequate mixing effect.